(A) Field of the Invention
The present invention relates to a method and an apparatus for semiconductor wafer transfer, and more specifically, to a method and an apparatus for semiconductor wafer transfer between a pod and a cassette.
(B) Description of Related Art
During the semiconductor manufacturing process, a cassette with wafers is disposed in a pod to prevent contamination especially while wafers are transferred between two processes. The pod may be equipped with a tag recording the lot number of the wafers for use in factory automation.
For some processes, the cassette has to withstand exposure to chemicals, e.g., photoresist removing in acid solution. During chemical mechanical polishing (CMP), e.g., in Mirra Mesa (a CMP tool of Applied Materials, Inc.), the wafers need to be put in water before being polished in order to avoid slurry crystallization. Accordingly, the wafers must be transferred from the original cassette to another cassette with different material, e.g., Teflon™ (polytetrafluoroethylene (PTFE)), before being processed.
Traditionally, in order to transfer the wafers in a pod to a cassette, the pod has to be unloaded by an auto loader unit (ALU) first, and then the user uses a suck pen to transfer wafers one by one. This requires a large amount of time and manpower.
In order to improve the efficiency of wafer transfer, a cassette-to-cassette (C-to-C) apparatus that can transfer a plurality of wafers at a time (batch wafer transfer) has been proposed. As shown in FIG. 1, a pod 11 is put on a platform 12 of an ALU 10. Generally, the pod 11 is composed of a base 112 and a transparent housing 111. A cassette 13 is placed on the base 112 and in the transparent housing 111. Two sides of the transparent housing 111 are equipped with handles 114, so that the user can take the pod 11 easily. Then, the cassette 13 with wafers is unlocked and unloaded from the pod 11 as shown in FIG. 2. The cassette 13 placed on the base 112 is lowered to the bottom of the ALU 10, while the transparent housing 111 is still on the platform 12. As a result, the user can take out the cassette 13 from the base 112. As shown in FIG. 3, the cassette 13 and an empty cassette 21 are placed on a C-to-C apparatus 20, and the openings of the cassette 13 and the empty cassette 21 face each other for wafer transfer therebetween. In this example, the cassette 21 is made of Teflon™ for CMP processing. A transfer robot 22 of the C-to-C apparatus 20 moves left so as to push the wafers in the cassette 13 toward the cassette 21. Accordingly, the wafers in the cassette 13 are transferred to the cassette 21 horizontally.
The traditional wafer transfer method between cassettes has many shortcomings. Because both the ALU and C-to-C apparatus need to be manually handled by the user, manpower is unavoidably wasted. In addition, the user needs to move the cassette including wafers from the ALU to C-to-C apparatus; during this period the wafers are not protected by the pod housing and are unavoidably exposed to air. Under such a situation, the wafers may be contaminated, damaged or otherwise scrapped due to incaution of the user.